The present invention relates to a roller which serves to press a sheet against a heating surface and which has a textured surface.
Rollers of the above-indicated kind are used in a fixing device which is incorporated in a copier and in which a toner image which has been developed with a developing liquid is fixed on a paper sheet by the application of heat. Such a fixing device is, for example, provided with a curved heating plate or heating surface, and the copy sheet carrying the liquid-developed toner image is guided through the fixing device and passed over the heating plate by means of a guide roller, i.e., a roller for pressing the copy sheet against the heating plate. The heating plate or heating surface, is heated by a correspondingly-shaped heating apparatus.
A thermal fixing station is used in electrophotographic copiers, in which a charge image produced on a photoconductor layer is developed to form a toner image, with the aid of a developing liquid containing toner pigments which are deposited on the charge image. The toner image is thereafter transferred to a copy base, for example, a paper sheet. The copy base, which carries the toner image and is moist with developing liquid, is introduced into the thermal fixing device, where the toner image is fixed on the paper and rendered smear-resistant by the application of heat.
German Offenlegungsschrift No. 25,039,642 discloses a device for fixing a toner image with the aid of heat. This fixing device is installed in an electrophotographic copier using a liquid developer. A heating apparatus is arranged in the fixing device. The liquid-developed copy sheet, which has the toner image on its upper surface, is conveyed past this heating apparatus, part of a heating surface of which is curved. The copy sheet contacts the curved heating surface with its reverse side and slides over the surface so that the toner image is fixed by heat. A roller is arranged at a short distance from the heating surface and is brought into contact with the surface of the sheet carrying the toner image, in order to convey the sheet to the curved area of the heating surface and transport it over the heating surface. The roller has a knurled peripheral surface and its distance from the bent or curved surface area of the heating surface is greater than the thickness of the sheet.
In copiers of the prior art, in which liquid developers are employed, knurled anodized aluminum rollers are used to press the copy base against the heating plate. If a developing liquid which deposits a greater amount of toner than conventional developing liquids is used in such a copier, it may happen that the still-soft toner is dragged along by the small knobs of the knurled aluminum roller and is retransferred to the copy base, after a complete revolution of the aluminum roller. The known knurled pressure rollers must be replaced after a particular operating time, since, on the one hand, too much toner settles in the grooves between the knobs of the knurled surface and soils the copy bases and, on the other hand, the knobs wear out, so that the copy base is no longer perfectly pressed against the heating surface, and sufficient fixing is thus no longer ensured. Replacing the pressure roller involves a prolonged downtime of the copier.